Loss of CD4+ T cell number or function may render individuals susceptible to opportunistic infection such as P. carinii pneumonia (PCP). This is exemplified by Human Immunodeficiency Virus infection, but also complicates other diseases in which normal immune cell function is perturbed. Although both observational and experimental data indicate that the CD4+ T cell is a critical determinant of the outcome of infection with Pneumocystis, the precise manner in which this effect is exerted is less clear. In addition to direct effector function, CD4+ T cells influence the function of other cells such as B cells, macrophages and cytolytic T cells through both soluble mediators as well as via direct intercellular interactions. Derangement in any of these elements of the immune response may lead to increased susceptibility to opportunistic infection. T cell activation is determined not only by engagement of the T cell antigen receptor, but also by theengagement of other receptors, termed costimulatory receptors. CD2 and CD28 are two well-described costimulatory molecules. Mice deficient in CD2 have little discernible phenotype on examination of T cell function. T cells from CD28-deficient mice have reduced proliferative responses and cytokine secretion. as well as impaired survival, but previously have not been noted to be susceptible to opportunistic infection. We have generated mice deficient in both CD2 and CD28. These mice have a profound defect in T cell activation despite normal lymphocyte numbers and distribution. Furthermore, the CD2/CD28 double deficient mice spontaneously develop and succumb to infection with P. carinii. Mice deficient only in CD28 do not develop P. carinii following co-housing with infected animals, yet are susceptible if inoculated intratracheally. The defined genetic defect that results in defective T cell function despite normal T cell number provides us with a novel opportunity to examine the role of T cells response to P. carinii. Furthermore, the differential susceptibility of CD28-deficient mice to naturally acquired infection vs direct inoculation allows us to examine how route of infection influences the host response. To address this, we propose the following two specific aims: 1 ) Characterize the response of costimulation deficient mice to infection with Pneumocystis carinii. The experiments proposed in this aim will provide quantitative data defining the susceptibility of the costimulation deficient mice and the characteristics of the immune response mounted against them. 2) Determine if reconstitution of specific cellular elements enable clearance of P. carinii infection in costimulation deficient mice. In this aim we will restore specific aspects of the host response by adoptive transfer into the costimulation deficient mice to dissect what required elements of the host response are lacking in the costimulation deficient mice. Although the observation of severe P. carirzii pneumonia in the double knockout mice was unexpected, these mice provide us with a powerful tool to examine the role of costimulatory molecules in defense against this important opportunistic pathogen.